Support writable foreign tables.

This patch adds the core-system infrastructure needed to support updates
on foreign tables, and extends contrib/postgres_fdw to allow updates
against remote Postgres servers.  There's still a great deal of room for
improvement in optimization of remote updates, but at least there's basic
functionality there now.

KaiGai Kohei, reviewed by Alexander Korotkov and Laurenz Albe, and rather
heavily revised by Tom Lane.

src/backend/optimizer/plan/createplan.c

@@ -20,6 +20,7 @@
 #include <math.h>
 
 #include "access/skey.h"
+#include "catalog/pg_class.h"
 #include "foreign/fdwapi.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
@@ -4695,7 +4696,8 @@ make_result(PlannerInfo *root,
  * to make it look better sometime.
  */
 ModifyTable *
-make_modifytable(CmdType operation, bool canSetTag,
+make_modifytable(PlannerInfo *root,
+				 CmdType operation, bool canSetTag,
 				 List *resultRelations,
 				 List *subplans, List *returningLists,
 				 List *rowMarks, int epqParam)
@@ -4703,7 +4705,10 @@ make_modifytable(CmdType operation, bool canSetTag,
 	ModifyTable *node = makeNode(ModifyTable);
 	Plan	   *plan = &node->plan;
 	double		total_size;
+	List	   *fdw_private_list;
 	ListCell   *subnode;
+	ListCell   *lc;
+	int			i;
 
 	Assert(list_length(resultRelations) == list_length(subplans));
 	Assert(returningLists == NIL ||
@@ -4746,6 +4751,53 @@ make_modifytable(CmdType operation, bool canSetTag,
 	node->rowMarks = rowMarks;
 	node->epqParam = epqParam;
 
+	/*
+	 * For each result relation that is a foreign table, allow the FDW to
+	 * construct private plan data, and accumulate it all into a list.
+	 */
+	fdw_private_list = NIL;
+	i = 0;
+	foreach(lc, resultRelations)
+	{
+		Index		rti = lfirst_int(lc);
+		FdwRoutine *fdwroutine;
+		List	   *fdw_private;
+
+		/*
+		 * If possible, we want to get the FdwRoutine from our RelOptInfo for
+		 * the table.  But sometimes we don't have a RelOptInfo and must get
+		 * it the hard way.  (In INSERT, the target relation is not scanned,
+		 * so it's not a baserel; and there are also corner cases for
+		 * updatable views where the target rel isn't a baserel.)
+		 */
+		if (rti < root->simple_rel_array_size &&
+			root->simple_rel_array[rti] != NULL)
+		{
+			RelOptInfo *resultRel = root->simple_rel_array[rti];
+
+			fdwroutine = resultRel->fdwroutine;
+		}
+		else
+		{
+			RangeTblEntry *rte = planner_rt_fetch(rti, root);
+
+			Assert(rte->rtekind == RTE_RELATION);
+			if (rte->relkind == RELKIND_FOREIGN_TABLE)
+				fdwroutine = GetFdwRoutineByRelId(rte->relid);
+			else
+				fdwroutine = NULL;
+		}
+
+		if (fdwroutine != NULL &&
+			fdwroutine->PlanForeignModify != NULL)
+			fdw_private = fdwroutine->PlanForeignModify(root, node, rti, i);
+		else
+			fdw_private = NIL;
+		fdw_private_list = lappend(fdw_private_list, fdw_private);
+		i++;
+	}
+	node->fdwPrivLists = fdw_private_list;
+
 	return node;
 }
 

src/backend/optimizer/plan/planner.c

@@ -571,7 +571,8 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 			else
 				rowMarks = root->rowMarks;
 
-			plan = (Plan *) make_modifytable(parse->commandType,
+			plan = (Plan *) make_modifytable(root,
+											 parse->commandType,
 											 parse->canSetTag,
 									   list_make1_int(parse->resultRelation),
 											 list_make1(plan),
@@ -964,7 +965,8 @@ inheritance_planner(PlannerInfo *root)
 		rowMarks = root->rowMarks;
 
 	/* And last, tack on a ModifyTable node to do the UPDATE/DELETE work */
-	return (Plan *) make_modifytable(parse->commandType,
+	return (Plan *) make_modifytable(root,
+									 parse->commandType,
 									 parse->canSetTag,
 									 resultRelations,
 									 subplans,
@@ -2035,6 +2037,15 @@ preprocess_rowmarks(PlannerInfo *root)
 		if (rte->rtekind != RTE_RELATION)
 			continue;
 
+		/*
+		 * Similarly, ignore RowMarkClauses for foreign tables; foreign tables
+		 * will instead get ROW_MARK_COPY items in the next loop.  (FDWs might
+		 * choose to do something special while fetching their rows, but that
+		 * is of no concern here.)
+		 */
+		if (rte->relkind == RELKIND_FOREIGN_TABLE)
+			continue;
+
 		rels = bms_del_member(rels, rc->rti);
 
 		newrc = makeNode(PlanRowMark);

src/backend/optimizer/prep/preptlist.c

@@ -6,7 +6,8 @@
  * For INSERT and UPDATE queries, the targetlist must contain an entry for
  * each attribute of the target relation in the correct order.	For all query
  * types, we may need to add junk tlist entries for Vars used in the RETURNING
- * list and row ID information needed for EvalPlanQual checking.
+ * list and row ID information needed for SELECT FOR UPDATE locking and/or
+ * EvalPlanQual checking.
  *
  * NOTE: the rewriter's rewriteTargetListIU and rewriteTargetListUD
  * routines also do preprocessing of the targetlist.  The division of labor